1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a fax, a multi-function apparatus which includes at least some of these functions, etc., with a writing laser power control.
2. Discussion of the Background Art
An image forming apparatus with a writing laser power controller is disclosed in, for example, JOP2-79060, JOP5-197263 and JOP58-121145. Such disclosed image forming apparatus with a writing laser power controller have a function of multi-resolution and can change writing resolution by changing a frequency of a pixel clock and a rotating speed of a polygon mirror. Or to put it another way, in such apparatus a scanning speed of exposure to light and a rotating speed of a photoconductor can be changed.
In such apparatus, energy of the exposure to light is maintained uniformly. To be more specific, when a rotating speed of a photoconductor is reduced by a half, for example, for changing a resolution, the amount of light for exposure is also reduced by a half. And when only the scanning speed of exposure to light is changed without changing the rotating speed of the photoconductor, the amount of light for exposure is not changed because energy of the exposure to light is not changed.
A copy machine is required to form a high quality image output for several different image types, e.g. a textual image and a photo image including halftones, and therefore, a copy machine may have a one dot multi-level image mode for an image forming function. A one dot multi-level image mode means that an image is formed of many dots and each dot is formed of multi-levels.
On the other hand, a printer is required to form a high quality output of a textual image, and therefore, a printer may have an output function by high resolution and binary dots. For example, for a normal mode resolution is 400 dpi, and for a high resolution mode resolution is 600 dpi, and the dots are binary dots.
And a multi-function apparatus which includes a copy function and a printer function may have an image forming function by the one dot multi-level and the binary dot image operations. However, in a typical multi-function apparatus, an energy of exposure to light is set for the one dot multi-level image to output a high quality image for a photo image including halftones.
FIG. 2 shows a relationship of energy of an exposure to light relative to a potential of a photoconductor. In forming an image by a one dot multi-level operation, an exposure potential is 1/5-1/10 of a charged potential. In FIG. 2, when the energy of the exposure to light is 4 erg/cm.sup.2 (point A), an exposure potential (-135V) is about 1/7 of a charged potential (-925V). Therefore, when the maximum energy of the exposure to light is controlled to 4 erg/cm.sup.2, the exposure potential is 1/5-1/10 of a charged potential.
However, an image forming apparatus which has an exposure system as just described has some problems. For example, in a multi-resolution image forming apparatus which has two resolution modes, e.g. a normal resolution mode (400 dpi) and a high resolution mode (600 dpi), when a frequency of a pixel clock is changed to 1.5 times of 400 dpi and set 600 dpi, energy of the exposure to light is maintained uniformly without changing an amount of light. Therefore, in solidly shaded areas the exposure potential is the same as for 400 dpi and the same image density can be output. However, in one dot, because an energy of the exposure is 2/3 of energy of the exposure for 400 dpi and a peak level is low, an isolated one dot image and a one dot line image are formed as low density and poor quality images.
To prevent this problem, there is a way to change a diameter of the light in the case of the 400 dpi and 600 dpi, and the amount of the light is changed to maintain uniformity. However, this operation cannot be executed easily because the operation of changing a diameter of the light system requires a high cost.
There is another problem when forming an image by binary dots also. An image forming operation is not stable by small fluctuations of amounts of light and characteristics of a photoconductor.